Are your HIV tests accurate?
HIV testing may seem very complex.
This is greatly due to the testing ‘Window Period‘. That irksome time from when a person is infected to when tests can actually detect a HIV infection.
HIV Signs and Symptoms
The HIV Window Period and Test Accuracy
HIV PEP Post Exposure Prophylaxis
- This has caused a lot of doubt and worry with regards to the accuracy of tests done.
- You might even be wondering right now how accurate your tests are.
- There is a lot of confusing information out there. Some people claiming a window period of 2 weeks some people even saying 10 years!
- This is because HIV testing technology has developed significantly since HIV was discovered in the 1980’s.
So a lot of information out there is out-dated.
Also, not the same standard of tests are available everywhere. This has caused different governments and different health authorities to develop different testing guidelines. So the reason why the Window Period is different in Massachusetts is different from Singapore is not because humans in the US react differently to HIV compared to humans in Singapore. It is because of local guidelines.
So what exactly if the HIV Window Period?
It depends a lot on WHICH TEST was done.
The following graph was prepared based on the latest information on Testing for the Diagnosis of HIV Infection.
Some points to note:
1. If you are within the first 10 days of possible HIV infection (aka the Eclipse Period), don’t bother doing any tests. There are NO tests that are accurate.
NB: If you are within the first 3 days of possible HIV infection please see us about PEP.
2. At day 10, the HIV-1 RNA PCR can detect HIV Viruses in the blood.
Remember that the RNA PCR test has a high chance of giving a false positive result.
If the RNA PCR test gives a report of less than 5000 viral copies per ml, it is likely a false positive.
Also, the viral load may drop to an undetectable level upon full sero-conversion.
This is because the body’s immune system may have suppressed the infection to a point where there is not enough HIV virus in the blood for the RNA PCR test to detect.
3. Between day 14 to day 20, the HIV P24 Antigen will start appearing in the blood.
This can be detected with 4th Generation tests (aka Combo tests).
This is why, although the official recommended Window Period for the Combo Test is 28 days, we are very confident when a patient tests negative at 2 weeks.Remember that the P24 Antigen level will peak at about day 28 and becomes undetectable at about 6 weeks from infection.
This does NOT mean the 4th Generation (Combo) test is inaccurate after 6 weeks because the Combo test also detects IgM and IgG Antibodies (See below)
4. Between day 20 and day 23, the IgM Antibody starts to become detectable in the blood.
This is only detectable by 3rd and 4th Generation HIV tests.
This is the other reason why even though the 4th Generation (aka Combo) test is supposed to be conclusive at 28 days, at 3 weeks when a patient is negative, we tell him/her it is as good as conclusive.
5. Between day 28 and 48, the IgG Antibody starts to become detectable in the blood.
This is detectable by 1st, 2nd, 3rd and 4th Generation HIV tests.
This will remain detectable in the blood for as long as a person lives. Therefore, doing a Combo test not only detects HIV infections that occurred recently, it also detects infections from years ago.
More things to note:
The recommended 1st line screening for HIV infection is the Combo test NOT the antibody test.So when you go for a HIV screening test, you should be asking for a Combo test. Take note that in Singapore the Combo test costs more than an antibody test.
The essential thing to remember about HIV testing is to get the RIGHT test done at the RIGHT time.